1. Field of the Invention
This invention deals with the techniques of bending glass sheets by passing them over a shaping bed composed of a series of shaping rods, disposed along a path having a profile curved in the direction of travel of the glass sheets. The invention is especially applicable to the production of curved and possibly also toughened automobile panes.
2. Description of the Related Art
The technique referred to above is known from French Patents FR-B-2 242 219 and FR-B-2 549 465 and consists of causing glass sheets, heated in a horizontal furnace, to pass between two layers of rollers--or other revolving elements--disposed along a curvilinear profile and passing through a final toughening zone. For the production of side window panes, opening roofs or other panes of cylindrical shape, the layers as a general rule are formed of straight cylindrical rods disposed along a circular profile. This technique makes possible a very high production capacity because, on the one hand, the glass sheets do not have to be widely spaced apart, it being possible for one glass sheet to enter the shaping zone without problem while the treatment of the preceding sheet is not yet finished and, on the other hand, if the length of the rollers allows, two or three glass sheets may be simultaneously treated abreast.
In the majority of cases, automobile panes have enamelled zones on the internal face, that is to say on the concave side; this enamel is of necessity deposited on that face of the glass sheets which faces upwards in the furnace, in order not to foul the conveyor of the furnace and, indirectly, the other glass sheets. Consequently, the path of the glass sheets must be ascending and the shaping bed has an upward concavity. In these circumstances, the glass sheet "climbs" by a step for each roller of the shaping bed.
To help the glass ascend this shaping bed, it has been proposed, in Patent FR-B-2 549 465, to use an assembly of upper rollers lining the shaping bed. These upper rollers are mounted with springs so as to prevent excessive bearing forces acting upon the glass, the objective being simply to obtain an assistance to the advance and in no way a rolling or pressing effect. However, this is possible only if the upper rollers are absolutely correctly adjusted in position, which presupposes a fairly complex machine. In European Patent Publication EP-B-260 030, it has also been demonstrated that these upper rollers can be eliminated if the speed of travel of the glass plates is high, more exactly at least 10 cm/second and preferably of the order of 15 to 18 cm/second.
By increasing the speed of passage, the time during which a given point of the glass sheet is not supported, because it is between two rollers, is reduced. For this reason, the risk of creating optical defects and bulges due to the formation of small waves is reduced. Everything happens as if the rollers or other equivalent revolving elements were closer together, whereas this physical closeness of the rollers is not permissible, essentially for technical limits resulting, notably, from the size of the pinions and other transmission mechanisms for the rotary movement and the minimum diameter necessary for preventing any deformation of the rollers.
The preferred speed of travel, of the order of 15 to 18 cm/second, also allows good contact between the glass and the rollers, by limiting the risk of slip of the glass relative to the rollers.
The increase in the speed of passage of the glass sheets is, however, limited by a second critical factor. Although an increase in output rates is desirable, the pane must not be allowed to leave the bending and toughening machine without being correctly toughened.
Toughening is carried out as the panes travel through the toughening zone, with glass sheets displaced at the same speed as in the bending zone. The degree of toughening is therefore a direct resultant of the speed of travel of the glass sheet, of the length of the toughening zone and of the temperature of the glass at its entry into the toughening zone.
To operate with a hotter glass in the toughening zone obviously implies a hotter glass in the bending zone, with an accentuation of the phenomena of deformation in the form of small waves referred to above. Furthermore, the length available for the toughening zone must be made shorter if the radius of curvature to be given to the glass sheet is reduced, since the discharge of the glass sheet from the bending/toughening machine must be carried out no later than after the glass sheets have travelled through a quarter of a revolution, and therefore before they return backwards. For a radius of curvature of 1 meter, it being understood that the length of the bending zone must of the order of 30 centimeters, the toughening zone can have at most a length of approximately 1.25 m.
In relatively standard blowing conditions and for a glass sheet of 3.2 m thickness, which must comply with the requirements of European Regulation No. 43 relating to the approval of safety glass and of the materials for panes intended to be fitted to motor vehicles and their trailers, the toughening stresses must be such that the pane exhibits, in the case of breakage, a number of fragments which, in any square of 5.times.5 cm, is neither less than 40 nor greater than 350 (this number raised to 400 for panes of a thickness less than or equal to 2.5 mm). Still according to these requirements, none of the fragments must be of more than 3.5 cm.sup.2, except possibly in a band of 2 cm width along the periphery of the pane and within a radius of 7.5 cm around the point of impact, and no elongate fragment of more than 7.5 cm length shall exist.
In order to satisfy these requirements, a glass sheet must remain in the toughening zone of the bending machine for a period of at least 5 seconds, which leads to a calculation of a limiting speed of passage of the glass sheets of 25 cm/second.
By comparison with the preferred speed of travel indicated earlier, this evidently gives a comfortable operating margin, and this requirement is effectively complied with for rectangular or essentially rectangular panes, for which the maximum distance between two points unsupported by the rollers corresponds to the distance between centers of these rollers. In contrast, if the pane has an oblique edge making an angle with the direction of travel of the glass sheets, this distance increases in inverse proportion to the cosine of this angle. With an edge at 30.degree., an identical support for the points of the edge would require a speed of travel of between 30 and 36 centimeters/second, and would therefore be unacceptable. Calculation shows that the limiting speed of 25 centimeters/second is reached as soon as the angle is less than 45.degree., which explains the appearance of defects of the scallop type at the edges. And it is self-evident that it is not possible to overcome this problem by modifying the position of the glass sheet relative to this direction of travel, because this direction also determines the principal direction of the curvature given to the sheet.